Hitherto, as a drive device commonly used in an industrial machine, and the like, a hydraulic drive device has been known. The hydraulic drive device includes a cylinder, a piston, a rod, a hydraulic pump, a valve, and a tank. In this device, a hydraulic oil discharged from the hydraulic pump is supplied to a head chamber or a rod chamber of the cylinder by the switching of the valve. A pressure of the hydraulic oil at that time allows the piston and the rod to move in a reciprocating manner.
Such a hydraulic drive device is used, for example, for a kneader which kneads rubber for a tire. Specifically, a drive portion (ram) which kneads rubber is connected to a top end of the rod, and the ram kneads rubber in accordance with a reciprocating movement of the piston and the rod.
Incidentally, a kneading quality in a kneader for a tire rubber depends on whether a ram is pushed onto rubber as a target constantly at a desired pressure and speed (uniformization of an agitation effect on rubber), and to achieve the same, in a hydraulic cylinder system, an extremely high pressure control performance is required.
Incidentally, in such a hydraulic drive device, depending on an operation situation of the ram, a volume expansion of the head chamber of the cylinder fails to be followed by a flow rate of the hydraulic oil supplied to the head chamber from the hydraulic pump, which may generate a negative pressure in the head chamber. In such a case, cavitation occurs in the head chamber, and hydraulic equipment and a hydraulic circuit may suffer from various troubles.
As a technique for preventing such a problem (negative pressure generated in the head chamber), a hydraulic drive device as described in Patent Document 1 has been known. The hydraulic drive device described in Patent Document 1 includes two respective hydraulic pumps which can supply a hydraulic oil independently of each other to a head chamber and a rod chamber of a cylinder. These hydraulic pumps are each controlled in an independent manner, thereby preventing a head pressure from becoming a negative pressure regardless of an operation situation of a load connected to a rod.
In the hydraulic drive device as described in Patent Document 1, the two pumps are each required to be controlled in an independent manner so that the control over the whole device becomes complicated.